The present invention relates generally to magnetic resonance imaging, and more specifically, to detecting and compensating for magnetic fields disturbances in a magnetic imaging device.
In magnetic resonance imaging (MRI), the protons of an imaged body are excited into resonance by a radio frequency field applied to the imaged object in the presence of a static magnetic field. The static magnetic field may be produced by a superconducting magnet having multiple coils of superconducting wire immersed in a cryogen and energized with an electrical current. Field strengths of several Tesla may be achieved with essentially no power consumption.
The frequency of the resonance of the protons of the imaged object excited by the radio frequency field is dependent on the strength of the magnetic field and certain characteristics of the protons.
As the protons precess in resonance, separate gradient magnetic fields of substantially smaller strength than the static magnetic field are applied to the imaged body to shift the phase and frequency of the resonance of the protons in accordance with each proton""s location within the imaged object. The combined signal produced by the resonating protons is then analyzed mathematically to produce an image of the imaged body along a xe2x80x9cslicexe2x80x9d through the imaged body.
The contribution of each resonating proton to the slice image is dependent of the phase and frequency of its resonance. If the static magnetic field is uniform, this phase and frequency will be dependent solely on the position of the protons in the gradient magnetic field. If the static magnetic field is not uniform, the apparent position of the protons, as determined by the phase and frequency of their resonance, will be shifted. This introduces artifacts or other distortions into the reconstructed image of the imaged body. The elimination of such artifacts requires that the static magnetic field used in MRI must be extremely uniform. Magnetic field homogeneities of less than a few parts per million over the imaging volume are required.
It follows that the static magnetic field also must be highly stable. The time required to collect the data for a single MRI slice image may be several minutes for certain imaging techniques. Mechanical disturbances of the magnet or magnet structures cause time-dependent changes in the magnetic field strength. Such mechanical disturbances cause magnetic field changes that may result in ghosting artifacts in MR imaging. Depending on the driving mechanical function, these disturbances may additionally result in vibration of said magnet structures resulting in oscillation of the magnetic field strength. The magnetic field changes may be spatially independent or may have additionally spatially-dependent terms such as a linear dependence on a given spatial axis or a higher order, such as a squared, dependence on a given spatial axis within the imaging volume of the magnet. Mechanical disturbances of the magnet may be caused by environmental disturbances such as building vibration or self-induced in the MRI system by pulsing of the magnetic field gradients during MR imaging.
It would therefore be desirable to improve the quality of a magnetic resonance image to reduce undesirable artifacts or distortion in the field due to varying magnetic field strength.
It is therefore one object of the present invention to determine magnetic field changes by detecting magnet structural motion or magnetic field changes and to actively cancel the magnetic field changes in the MRI device to improve the resulting image.
In one aspect of the invention, an apparatus for producing magnetic resonance images includes a first magnet portion and a sensor positioned at a pre-determined relationship with respect to the first magnet portion. The sensor generates a magnetic field change signal indicative of a magnetic field change generated at the first magnet portion. A control circuit is coupled to the sensor and generates a compensation signal in response to the magnetic field change signal. The compensation signal is used to actively cancel changes in the magnetic field.
In another aspect of the invention, an electromagnetic compensation coil may be used to receive the compensation signal to generate active canceling of the changed magnetic field. In yet another aspect of the invention, the compensation signal may be coupled to a transceiver circuit to change the center frequency of the transceiver circuit and therefore compensate the effect of magnetic field changes.
In yet another aspect of the invention, a method for compensating for magnetic field disturbances in a magnetic resonance imaging device having a first magnet portion comprising the steps of:
generating a magnetic field change signal indicative of a change in the magnetic field;
determining the compensation signal in response to the magnetic field change signal; and
actively canceling the change in the magnetic field in response to the compensation signal.
One advantage of the invention is that various types of sensors may be incorporated into the system in a redundant manner.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.